A silicone rubber is excellent in heat resistance, flame retardancy, chemical stability, weather resistance, radiation resistance, electrical characteristics, and the like, and is therefore used in various applications in a wide range of fields.
In particular, since the silicone rubber is physiologically inactive and has little response to body tissues when it is brought into contact with living bodies, it is therefore used as a material for medical instruments such as various catheters for medical uses.
A catheter for medical use is a tube which is inserted into a body cavity such as an abdominal cavity and a chest cavity, a tube cavity part such as the digestive tract and the urinary tract, blood vessels, or the like, and is therefore used for fluid drainage or for injecting and dripping of chemical liquids, nutritional agents, contrast agents, or the like. The catheter for medical use is required to have, in addition to biocompatibility, scratch resistance (tear resistance), kink resistance (tensile strength and compression permanent strain), transparency, flexibility (tensile elongation), and the like. Examples of the specific applications of the catheter for medical use include a drainage tube of an aspirator for removing drainage such as blood and pus, and a tube for nutrition intake after surgery such as percutaneous endoscopic gastrostomy (PEG). Further, in order to produce ultrafine tubular silicone rubber for a catheter, a silicone rubber composition which is a material of the silicone rubber is required to have extrusion moldability.
As the material of the catheter for medical use, in addition to a silicone rubber, soft polyvinyl chloride or the like is generally used. As compared with polyvinyl chloride, the silicone rubber is excellent in biocompatibility and flexibility, but is required to have a decreased compression permanent strain, which is indicative of a restoring force, and is also required to have a surface having improved strength such as tear strength and tensile strength, in particular, tear strength. If the tear strength is not sufficient, the catheter may be torn by a scratch by a knife, a needle, or the like during a treatment; and if the tensile strength and the compression permanent strain are not sufficient, the catheter bends and yields, and thus becomes obstructed (kinked). As a result, the flow of a fluid which needs to be discharged or a chemical liquid which needs to be injected in a catheter is interrupted.
Therefore, in order to improve the tear strength, the compression permanent strain, and the tensile strength of a silicone rubber (for example, PTLs 1 to 7), various methods have been proposed. Examples of the specific method for providing the silicone rubber with high tear resistance include the addition of an inorganic filler or the like and the increase in a crosslink density (distribution of an area having a high crosslink density and an area having a low crosslink density in a silicone fine particle system). The reason therefor is thought to be that the improvement of the tear resistance by increasing the crosslink density makes the area having a high crosslink density act as a resistance to tearing stress.
More specifically, in PTL 1, a curable silicone rubber composition is disclosed having an organopolysiloxane (raw rubber (A)) having a high viscosity and a low content of vinyl groups, which is blended with an organopolysiloxane (silicone oil (B)) having a low viscosity and a high content of vinyl groups, a vinyl group-containing organopolysiloxane copolymer (vinyl group-containing silicone resin (C)), an organohydrogen siloxane (crosslinking agent (D)), platinum or a platinum compound (curing catalyst (E)), and fine powder silica (filler (F)).
However, even though an organopolysiloxane having a high content of vinyl groups is used and blended in combination with an organopolysiloxane having a different content of vinyl groups as in PTL 1, it is possible to increase the tensile strength by increasing the cross-linking points, but there remains a problem in that a sufficient tear strength is not obtained and further, the compression permanent strain is not decreased.